Vacuum vapor deposition with control of elevation of metal melt

ABSTRACT

IN THE PROCESSES FOR FORMING PROTECTIVE COATINGS ON METAL SUBSTRATES, PARTICULARLY THE NICKEL-BASE AND COLBALTBASE SUPERALLOYS, BY MELTING A COATING MATERIAL TO CAUSE VAPORIZATION THEREOF, A MONOCHROMATIC LIGHT BEAM IN UTI-   LIZED IN SENSING AND CONTROLLING DISPLACEMENT OF THE SURFACE ELEVATION OF THE MELT.

April 13, 1971 R. D. HOUSE 3,574,650

VACUUM VAPOR DEPOSITION WITH CONTROL OF ELEVATION OF METAL MELT FiledMarch 15, 1969///lII/IIIIIIlI/IIIII/III/(IIIII/IIIIl/IIII/II/IIIIlIll/IIII/I'll!Ill/III/lI/I/IIIIIIIIIIIIIIIIIIIfl/Ill IIIIIIIIIIIIIIIIIIIIII"\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\s \\\\\\\\\\\vllm I iii]? INVENTOR. RANDOLPH D. HOUSE ATTORNEY United States PatentUS. Cl. 117-107 5 Claims ABSTRACT OF THE DISCLOSURE In the processes forforming protective coatings on metal substrates, particularly thenickel-base and cobaltbase superalloys, by melting a coating material tocause vaporization thereof, a monochromatic light beam is utilized insensing and controlling displacement of the surface elevation of themelt.

BACKGROUND OF THE INVENTION The present invention relates in general toa method for sensing changes in the relative location of a material andmore particularly relates to an electro-optical method for sensing andcontrolling the relative surface location of a light reflectivematerial. The invention is particularly useful in detecting andcontrolling the surface elevation of an incandescent metal melt as it isdepleted by evaporation in a vacuum vapor deposition process.

It is well known that the conventional nickel-base and cobalt-basesuperalloys do not in and of themselves exhibit sufficientoxidation-erosion resistance to provide component operating lives ofreasonable duration in the dynamic oxidizing environments such as thoseassociated with the operation of gas turbine engines. Accordingly, ithas been the usual practice to provide these alloys with a protectivecoating in such applications.

Although the aluminide coatings, such as that described in the patent toJoseph 3,102,044, have in the past displayed satisfactory performance,it is well known that these coatings, because of their dependence uponthe availability of substrate elements, often are characterized by acomposition less than optimum.

Many of the more advanced coatings developed for the next generation ofjet engines depend in the first instance on the deposition of a highmelting point coating alloy with a concurrent or subsequent reactionwith the substrate to attain the desired end composition, microstructureor adherence. These new alloys generally demand the application ofspecial coating techniques to provide the right species in the rightamounts on the surfaces to be protected.

Several coating compositions of current interest are described in detailin copending applications of the present assignee. Among thesecompositions is that hereinafter referred to as the FeCrAlY coating at anominal composition of, by weight, percent chromium, 15 percentaluminum, O.5 percent yttrium, balance iron, as discussed in thecopending application of Frank P. Talboom, r. et a]. entitled Iron BaseCoating for the Superalloys, Ser. No. 731,650, filed May 23, 1968.Another such composition is the CoCrAlY composition at about, by weight,21 percent chromium, 15 percent aluminum, 0.7 percent yttrium, balancecobalt.

The basic problems associated with the deposition of these coatingalloys relates to their high melting points and the ditficulty ofproviding the right amount of all of the alloy species in the coating asapplied. Satisfactory results have been attained through the use ofvacuum vapor deposition techniques, such as that suggested in the patentto Steigerwald 2,746,420. These processes, which have in the past beenprimarily directed toward the appliice cation of relatively lowtemperature materials of relatively simple composition are, in thepresent instance, characterized by extreme sensitivity to variations inthe process parameters and, accordingly, reproducibility as Well asprocessing expense are formidable problems.

A significant problem in existing vacuum vapor deposition processes hasbeen a lack of effective means for sensing and maintaining the moltensource pool at a constant height. It has been demonstrated that coatingefiiciency, composition and uniformity are very susceptible to poolheight changes. This is true not only with regard to relative changes inposition and spacing between source and substrate, but more importantly,with regard to relative changes in elevation of the molten pool withinthe crucible. Recently, several techniques have been developed toimprove the effectiveness of the basic process through the mechanism ofmonitoring the coating source material elevation. In one such method, aradioactive 1sotope is utilized as a source of radiation in a systemwherein the amount of radiation passing over or through the molten poolindicates the elevational location thereof.

SUMMARY OF THE INVENTION The present invention contemplates detectionand control of a displaceable material having reflective properties bythe utilization of a light beam of sufiicient intensity to be monitoredin reflection.

According to a preferred embodiment of the invention, a high intensitymonochromatic light beam such as a laser beam is utilized in a vaporvacuum deposition process and is focused on the surface of the moltensource metal pool at a predetermined angle of incidence. The reflectedbeam is refocused onto a photodetector which is sensitive to movementsof the beam caused by elevational displacement of the pool surface. Thephotodetector, in essence, monitors and maintains the evaporating moltenmetal at a constant elevation to ensure process uniformity andreproducibility.

BRIEF DESCRIPTION OF THE DRAWING An understanding of the invention willbecome more apparent to those skilled in the art by reference to thefollowing detailed description when viewed in light of the accompanyingdrawing, wherein is shown a schematic llustration, partially in section,of a vacuum vapor coatmg apparatus in accordance with this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In one particular embodiment asillustrated in the drawmg, there is shown a vacuum chamber 10 having anexit port 12 leading to a suitable high vacuum pump, preferably of thedilIusion type, for the rapid and continuous evacuation of the chamber.Located inside the chamber, there is shown an electron gun 14 forgenerating a beam of charged particles to impinge upon and vaporize aningot of source metal 16. It will be appreciated by those skilled in theart that the electron beam is suitably directed by conventional magneticdeflection pole pieces 18. Of course, the arrangement of the electronbeam gun within the vacuum chamber is a function of design. A 30kilowatt electron beam unit has been used to melt and vaporize the upperend of ingot 16 to create a molten source pool having a reflectingsurface 20. Satisfactory deposition rates have been achieved with a twoinch diameter ingot of a FeCrAly coating material, the depth of themolten pool usually being /4 /2 inch.

The ingot 16 is slidably received at its upper end by a fixed annularwater cooled copper crucible 22. The ingot is movable vertically by anactuator or motor means 24 which in turn is electrically controlled bymeans herein- 3 after described. The ingot 16 passes through a suitableheat resistant vacuum seal 26 in the chamber base.

A substrate 27, to be coated, is disposed within the vacuum chambervertically above the pool surface 20. Since the process is fundamentallyline-of-sight, the part is typically mounted to effect rotation aboutits longitudinal axis, usually utilizing a vacuum sealed pass-through(not shown) through the vacuum chamber to an external drive system. Ofcourse, more than one part may be coated at a time. In such a case, inorder to minimize non-uniformity of coating between each of theplurality of parts, each part is normally mounted in a plane of vaporisodensity or roughly along an arc defining a zone of constant vaporconcentration, the parts closest to the vertical passing through thecenter of the molten pool being located slight- 1y farther from the poolsurface than those positioned at an angle with respect to the saidvertical. Whether coating a single part or a plurality of parts however,each substrate is further positioned as close as possible to the surfaceof the molten source pool without being subjected to nudesirable coatingcontamination by splash from the pool. The substrate height varies witheach system but for a two inch diameter pool and a deposition rate ofabout 0.3 mil per minute with a FeCrAlY coating material, a mean heightof about inches has been found satisfactory.

As indicated previously, keeping the molten pool surface at a constantelevation is of great importance in maximizing coating efliciency,uniformity and composition. Although prior attempts to monitor andcontrol vapor source materials have been useful, it is believed that thepresent electro-optical displacement sensing system is distinct andadvantageous in accomplishing this end.

As shown in the drawing, there is provided a source of I high energymonochromatic light, such as a laser 28. In

practice, a 1.0 milliwatt helium-neon gas laser has been foundsatisfactory. The light beam, designated by the numeral 30, is projectedand focused to a small spot 31 on the reflecting surface of the moltenbody by appropriate source optics, such as by a microscope objectivelens 32 and a lano-convex lens 34. The optical components 32 and 34 areprotected against vapor fouling by an inwardly projecting tubularencasement 36 having a transparent window 38 which seals theaforementioned optics from the chamber. The window 38 is itselfprotected against fouling by suitable apparatus, such as a tube shield40 provided with an inert gas swee introduced through a gas inlet 42.The free end of the shield 40 is apertured at 44 with the opening justlarge enough to permit uninterrupted passage of the light beam. Therelative sizes of the optical and protective components can of coursevary. However, best results have been obtained by providing a largefocal distance between the lens 34 and the spot 31, and a relativelysmall aperture 44. For example, components giving a focal distance of 33inches with a beam diameter of /2 inch at the lens 34 and a tube shield15 inches long and apertured to closely circumscribe the beam at alocation approximately 16 inches from the spot 31 were found desirable.As will subsequently be understood, such a relation is advantageous inthat there is provided a large depth of field which reduces the effectof a changing spot size at the deflector face due to changes in theheight of the molten pool. Further, a long focal distance ensures agreater separative distance from the molten pool to minimize vaporcondensate contamination while a small aperture 44 corresponding to thesmall diameter beam reduces the gas load, on the vacuum system, imposedby the protective inert gas sweep.

The light beam reflected from spot 31 is intercepted and focused onto aspot 46 on the face of a light sensitive detector 48 by similarappropriate detector optics such as plane-convex lens 50 and microscopeobjective lens 52. The detector optics are protected against vaporfouling in the same way as the source optics. There is provided anencasement 54 having a window 56, the window being protected by an inertgas sweep in a tube shield 58 with a gas inlet 60 and aperture 62. Thedetector 48 is preferably comprised of matching photo electric cells 64,disposed within the focal plane of the reflected beam and separated by anarrow horizontal gap 66, with each cell connected to appropriateelectrical lead lines 68. In practice, a photodetector comprised of atwo element silicon Schottky barrier photodiode has been foundsatisfactory. The detector 48 is preceded by a narrow bandpass filter 70which has its bandpass centered on the laser wavelength, and which ispreferably a narrow bandpass dielectric film optical interferencefilter. It can be seen that as the source material is depleted byevaporation, the pool surface 20 is lowered in elevation, thus alteringthe vertical and horizontal position of the spot 31 on the pool surfacewhere reflection occurs.

The component of motion of the spot 31 along a line normal to theotpical axis 23 of the detector optic and lying in a vertical planecontaining the optical axis, changes the angular position of the spotwith respect to the axis of the detector optics. The spot 46 isconcomitantly displaced from a position of balance in relation to thegap 66 to a position of imbalance whereby the spot impinges more fullyon one photo cell 64 than the other, thus causing electricallyunbalanced signals to be generated in the lead lines 68. By connectingthe lead lines from opposite sides of the detector 48 to a signalprocessor 72. in turn connected to the actuator 24, it will beappreciated by those skilled in the art that the photo cells 64 may beemployed as the active elements of a Wheatstone bridge circuit includingin the signal processor whereby unbalancing of electrical signals can beutilized to cause the actuator to move the ingot 16 in the appropriatedirection until balance is achieved.

In the described system, it is recognized that the surface of the moltenmetal pool in the vicinity of the small reflecting spot 31 iscontinually tilting at random by virtue of surface waves traversing thepool which constantly alter the direction of the reflected beam. Inorder to avoid excessive sensitivity to the tilting of the reflectingsurface, the detector optics 50, 52 are positioned so that thereflecting spot 31 and the photoelectric detector 48 are at theconjugate foci thereof. In this way, even though the surface waveamplitudes may become large enough to cause the reflected beam to miss,at times, the detector optics entirely, the system still functionseffectively because there results a periodic sweeping of the beam acrossthe aforementioned detector optics. At each sweep, the detectors produceoutput pulses the relative magnitudes of which are dependent upon theheight of spot 31 during the brief interval that the reflected beam isintercepted by the detector optics. The signal processor time averagesthe pulses and provides a control signal proportional to the diflerencebetween a desired control height and the time average height of themolten surface.

In order to maintain a high sensitivity to pool height changes, theincident beam 30 is preferably projected at a relatively large angle ofincidence, 6. It will be appreciated that since the sensitivity to poolheight is proportional to twice the sine of the angle 0, an angle ofincidence in the range of 30 to is preferable, although the system isoperable at angles as small as 10. An angle of incidence ofapproximately 70 however is found to give best results in the presentsystem since 70 affords a satisfactory compromise between optimizingsensitivity while avoiding interference by the edges of the cruciblewith the incident and reflected beams.

A conventional beam splitter 74 and viewing screen with integral reticle76 are illustrated as providing a means for effecting visual observationof the elevation of the pool surface.

It is to be understood that various modifications can be made withoutdeparting from the spirit of the present invention. It is recognized,for example, that while high intensity monochromatic electromagneticradiation is necessary for incandescent targets such as high meltingpoint molten metals, that polychromatic radiation may be utilized onbodies, molten or solid, which are held at lower temperatures 30 long asthey are sufliciently reflective. It is similarly recognized that thebody to be monitored can have reflective properties without inherentlypossessing a surface that is effectual as a reflector. In other words,the technique is also operable with a relatively non-reflective bodywhich is given reflective properties by having an auxiliary reflectingsurface element provided thereon.

What has been set forth above is intended primarily as exemplary toenable those skilled in the art in the practice of the invention and itshould therefore be understood that within the scope of the appendedclaims, the invention may be practiced in other ways than asspecifically described.

What is claimed is:

1. In the processes for forming protective coatings of metallic materialon a substrate by vacuum vapor deposition, the improvement whichcomprises:

melting metallic coating material in a crucible to form a molten pool,

evaporating molten pool, thereby causing a lowering of the pool surfaceelevation,

establishing an incident beam of monochromatic light on said molten poolsurface, said incident beam being focused onto a spot thereon, focusing,by optical means, the reflected beam onto a photodetector, said spot andsaid photodetector being located at the conjugate foci of the opticalmeans whereby changes in the molten surface elevation are detected, andgenerating signals in said photodetector proportional to said detectedchanges to raise said molten pool to maintain its surface at a constantelevation. 2. The invention of claim 1 wherein the angle of incidence ofthe beam is approximately 70.

3. In the processes for forming protective coatings of metallic materialon a substrate by vacuum vapor deposition, the improvement whichcomprises:

feeding an ingot of coating material upwardly through an annularcrucible; melting the upper end of said ingot to form an evaporatingpool having a surface level within said crucible;

establishing an incident beam of monochromatic light on said molten poolsurface, said incident beam being focused to a spot thereon;

detecting changes in the position of said spot, and

generating signals proportional to said detected changes to causemovement of said ingot to maintain said molten pool surface at aconstant elevation.

4. The invention of claim 3 wherein changes in the position of said spotare detected by focusing, by optical means, the reflected light beamonto a photodetector with said spot and said photodetector being at theconjugate foci of the optical means.

5. The invention of claim 4 wherein the angle of incidence of the beamis approximately References Cited UNITED STATES PATENTS 2,746,420 5/1956Steigerwald 1l849X 3,086,889 4/1963 Strong 117107X 3,114,790 12/1963Hanks 118-9X 3,117,024 1/1964 Ross l18-9X 3,123,712 3/1964 SpOoner250--43.5 (L)X 3,170,383 2/1965 Hunt 118-9UX 3,244,557 4/1966 Chiou eta1 1187UX 3,316,468 4/1967 Hanks 118-9X 3,347,701 10/1967 Yamagishi eta1. 11849.1X 3,373,278 3/1968 Cilyo 250-495(8) ALFRED L. LEAVI'I'I,Primary Examiner I. R. BATTEN, I R., Assistant Examiner US. Cl. X.R.

qgxgg UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent3l574.650 Dated @ril 13 1971 Inventoz-(s) landolph D. House It iscertified that error appears in the above-identified patent and thatsaid Letter-a Patent are hereby corrected as shown below:

Column 6, line 4, "rating pool" should read --rating molten pool--Signed and sealed this 13th day of July 1971 (SEAL) Attest:

EDWARD M.FLETCHER,JR. WILLIAM E. SGHUYLER, J Attesting OfficerCommissioner of Patent

